Secure tags are used for a number of different purposes; a primary purpose being preventing, detecting, and/or deterring counterfeiting.
One type of secure tag that has recently been developed is based on small particles of a rare earth doped host, such as glass. This type of secure tag is described in US patent application No. 2004/0262547, entitled “Security Labelling,” and US patent application No. 2005/0143249, entitled “Security Labels which are Difficult to Counterfeit”, both of which are incorporated herein by reference.
These rare earth doped particles (hereinafter “RE particles”) can be applied to valuable items in different ways. For example, the secure tags can be incorporated in fluids which are applied (by printing, spraying, painting, or such like) to valuable items, or incorporated directly into a substrate (paper, rag, plastic, or such like) of the valuable items.
In response to suitable excitation, RE particles produce a luminescence spectrum having narrow peaks because of the atomic (rather than molecular) transitions involved. Luminescence is a generic term that relates to a substance emitting optical radiation in response to excitation, and includes photoluminescence, such as fluorescence and phosphorescence.
Fluorescent materials (dyes and pigments) tend to have a decay lifetime of 10−9 to 10−7 seconds (1 to 100 nanoseconds). The fluorescence disappears very quickly after excitation ceases. Thus, detecting fluorescence is typically performed simultaneously with excitation.
Phosphorescent materials (dyes and pigments) tend to have a decay lifetime of 10−3 to 100 seconds. Although detecting phosphorescence can be done simultaneously with excitation, it is also possible to measure phosphorescence after the excitation is removed, thereby adding to the security of a phosphorescent secure tag.
It would be desirable to have secure tags that include multiple levels (or layers) of codes. For example, it would be desirable to have a secure tag that luminesces at one or more fixed wavelengths to indicate that the secure tag is genuine, or owned by a particular company (the first level code); but also luminesces at one or more additional wavelengths to indicate a certain feature or quality of the substrate the secure tag is applied to (the second level code). Thus, different secure tags from the same company would have the same fixed wavelengths but different additional wavelengths.
The difficulty with achieving multiple levels of codes is that the luminescence from a secure tag is determined by the electronic states within that secure tag. When dopants are added to introduce luminescence at additional wavelengths, the luminescence from the fixed wavelengths change unpredictably. Thus, introducing a new dopant to provide a new second level of code may change the first level of code, thereby ruining the multi-level coding arrangement. This unpredictability means that the only reliable way to ensure that a new dopant does not affect the first level code (and thereby provide consistent multiple levels of codes) is by trial and error, which is expensive. If a large number of different second level codes are desired, it may not be feasible to use the trial and error approach.